Electrophotographic printers wherein a laser scan line is projected onto a photoconductive surface are well known. In the case of laser printers, facsimile machines, and the like, it is common to employ a raster output scanner (ROS) as a source of signals to be imaged on a pre-charged photoreceptor (a photosensitive plate, belt, or drum) for purposes of xerographic printing. The ROS provides a laser beam which switches on and off as it moves, or scans, across the photoreceptor. Commonly, the surface of the photoreceptor is selectively imagewise discharged by the laser in locations to be printed white, to form the desired image on the photoreceptor. The on-and-off control of the beam to create the desired latent image on the photoreceptor is facilitated by digital electronic data controlling the laser source. A common technique for effecting this scanning of the beam across the photoreceptor is to employ a rotating polygon surface; the laser beam from the ROS is reflected by the facets of the polygon, creating a scanning motion of the beam, which forms a scan line across the photoreceptor. A large number of scan lines on a photoreceptor together form a raster of the desired latent image. Once a latent image is formed on the photoreceptor, the latent image is subsequently developed with a toner, and the developed image is transferred to a copy sheet, as in the well-known process of xerography.
FIG. 1 shows the basic configuration of a scanning system used, for example, in an electrophotographic printer or facsimile machine. A laser source 10 produces a collimated laser beam 12 which is reflected from the facets of a rotating polygon 14. Each facet of the polygon 14 in turn deflects the laser beam 12 to create an illuminated beam spot 16 on the pre-charged surface of photoreceptor 18. The system may further include additional optical elements such as focusing lenses 15. The energy of the beam spot 16 on a particular location on the surface of photoreceptor 18, corresponding to a picture element (pixel) in the desired image, discharges the surface for pixels of the desired image which are to be printed white. In locations having pixels which are to be printed black, the beam 12 is at the moment of scanning shut off so the location on the surface of photoreceptor 18 will not be discharged. It is to be understood that grey levels are imaged in like manner by utilizing exposure levels intermediate between the "on" and "off" levels. Thus, digital data input into laser source 10 is rendered line by line as an electrostatic latent image on the photoreceptor 18.
When the beam spot 16 is caused, by the rotation of polygon 14, to move across photoreceptor 18, a scan line 20 of selectively discharged areas results on photoreceptor 18. In FIG. 1, the photoreceptor 18 is shown as a rotating drum, but those skilled in the art will recognize that this general principle, and indeed the entire invention described herein, is applicable to situations wherein the photoreceptor is a flat plate, a moving belt, or any other configuration. The surface of photoreceptor 18, whether it is a belt or drum, moves in a process direction;the motion of spot 16 through each scan line 20 is transverse to the process direction. The periodic scanning of beam spot 16 across the moving photoreceptor 18 creates an array of scan lines 20, called a raster 22, on the photoreceptor 18, forming the desired image to be printed. In real-world situations, such a configuration will typically further include any number of lenses and mirrors to accommodate a specific design.
In order for the electrostatic latent image to be successfully rendered on the photoreceptor, it is necessary that the series of scan lines 20 forming the raster 22 are properly aligned and consistently spaced from one another. The signals creating each scan line are created by the pattern of the scanning laser being modulated (turned on and off, or otherwise varied in intensity, selectively) as the beam spot 16 moves across the photoreceptor 18. Naturally, for a coherent image to be created on the photoreceptor, the time-coordination of the modulation of the beam 12 must be precise with regard to the location of the beam spot 16 on the photoreceptor 18 at any given time. When the beam spot 16 is located at a position on the photoreceptor 18 corresponding to a particular pixel forming the desired image, there must be certainty that the correct signal is output from laser source 1(::1. As the modulation of the beam spot is dictated by digital electronic data controlling the laser source 10, there must be close coordination between the laser source and the motion of the polygon surface and the photoreceptor.
This problem of coordination of data with a position of pixels in a scan line forming a raster is familiar both in the art of electrophotography and the art of television. In the electrophotographic context, various electronic or electro-mechanical schemes have been provided in the past for effecting this coordination. One of many examples of such a system is U.S. Pat. No. 4,279,002 to Rider, assigned to the assignee of the present application.
An optical element may be disposed between the light source of a scanning system and the photoreceptor surface to coordinate the imagewise digital data with the motion of a beam spot through a scan path across a photoreceptor surface. Various optical elements for use in such a context are known, although such optical elements are usually provided for purely optical correction of data scanned on the photoreceptor. U.S. Pat. No. 4,804,980 to Prakash et al. discloses an aspheric lens to be placed along the scan path of a beam spot in an electrophotographic printer. The lens exhibits varying optical power as a function of location along the longitudinal axis of the lens. The lens provides correction of tilt error and scan bow error of the beam spot through the scan path. U.S. Pat. No. 4,866,459 to Tokita et al. similarly discloses a scanning system with a lens system for the correction of images scanned onto the photoreceptor. UK Patent Publication 2 229 281A discloses an optical scanner utilizing a cylindrical lens for skew and bow correction of the beam on the photoreceptor.
U.S. Pat. No. 5,043,744 to Fantuzzo et al., assigned to the assignee of the present application, discloses an apparatus for monitoring and controlling the motion of a beam spot across a photoreceptor. The apparatus includes two photodetectors, one each disposed adjacent the photoreceptor at the start and end of the scan path, and adapted to detect an optical pattern caused by the reflection of the beam from timing marks on the moving photoreceptor. The configuration of the timing marks on the photoreceptor is adapted to permit monitoring of the "tilt" of the scan lines, i.e., the relative positions of the start and end of the scan line relative to the direction of motion of the photoreceptor.